The subject matter of the present invention relates generally to metal detectors, and in particular to induction balance type metal detectors having the ability to discriminate between different types of metal objects. The metal detector of the present invention is of great sensitivity and is especially useful in finding coins and other small metal objects buried in the ground or located in water and other inaccessible places.
Induction balance metal detectors have been provided with a means for discriminating between ferrous mineral soil and a metal object, as discussed on pages 93 to 96 of THE ELECTRONIC METAL DETECTOR HANDBOOK, by E. S. LeGaye, published in 1969 by Western Heritage Press of Houston, Texas. However, previously such induction balance metal detectors have not been provided with amplitude discrimination between different types of metal objects, such as valuable coins and worthless bottle caps or can pull tabs. The metal detector of the present invention is capable of discriminating between different types of metal objects having phase angles between about -90.degree. and -180.degree. relative to the received signal component produced by magnetic mineral soil, and in addition avoids false readings by employing inverse discrimination. Inverse discrimination as hereafter discussed refers to the fact that in the present circuit, coins and other highly conductive targets which are sought to be detected reduce the amplitude of the received coil signal below its normal "audio threshold" level while all other targets increase the amplitude of such received signal above the audio threshold level. Such inverse discrimination would ordinarily provide an undesirable negative indication of the coins by causing the detector to produce no sound or other audio output indication when the coins are detected. This problem is avoided by providing the detector with an amplitude comparator having one input connected to the demodulated output of the receive coil and another input connected to an audio pulse generator synchronized to the audio modulation of the R. F. oscillator signal so that the comparator produces an audio output signal only when the received signal is of less amplitude than the audio pulse. As a result, the no sound negative indication of a target is converted into a sound output positive indication of such target.
Another induction balance metal detector referred to as a phase readout gradiometer is shown on page 62 of the August 1974 issue of Western Treasures magazine, which employs a phase comparator for comparing the phase of the received output signal of the receiver coil with the phase of the oscillator signal connected to the transmit coil and displays the relative phase angle of the received signal on a meter. Thus the output of the phase comparator is an output pulse whose width varies with the phase of the received signal and whose amplitude apparently remains substantially constant. Unlike the inverse discriminator detector of the present invention, it does not employ a voltage comparator to compare the amplitude of the received signal with the amplitude of reference pulses to produce an audio output signal only when the received signal amplitude is less than that of such standard pulses. In addition, the phase readings provided on the meter of this instrument are unreliable in ferrous mineral soil because it shifts the phase of the received signal. The detector of the present invention avoids these problems as hereafter discussed.
Ferrous mineral soil changes the amplitude of the residual signal produced in the receive coil when no target is present, and may detune the detector so that it is no longer at the audio threshold level. As a result the sensitivity of the detector is decreased making it difficult to detect metal objects. Ferrous mineral soil also changes the phase of the residual signal produced in the receive coil when no metal target is present, and may therefore affect the discrimination capability of the detector so that certain metal objects produce a receive coil amplitude response which is positive at times but negative at other times depending on the amplitude of the mineral soil component of the receive coil output signal. These problems caused by mineral soil are overcome in the detector circuit of the present invention by using a tuning control which adjusts the amplitude of the residual receive coil signal along an axis having the same phase angle as the signal produced by the ferrous mineral soil. In addition, a trimmer capacitor is connected between the transmit coil and the receive coil so that it can also be used to adjust the amplitude of the residual receive coil signal produced when no metal object is present. As a result, the effect of ferrous mineral soil can be completely eliminated by adjustment of the trimmer capacitor and the tuning control so that the residual signal at the output of the receive coil is maintained at the audio threshold level and at the correct phase angle regardless of the type of soil encountered.